Pharmacokinetics practice questions
Fifteen questions on half-life, clearance, volume of distribution, dosing and drug interactions, with the working shown. Pick an answer to see the explanation.
Written by the Quizard team. Last updated
Most pharmacokinetics questions come down to a few relationships. If you can say what happens to half-life when clearance falls, or to steady state when the dose doubles, you can reason through questions you have not seen before.
| Quantity | Relationship |
|---|---|
| Volume of distribution | Vd = dose ÷ plasma concentration |
| Half-life | t½ = 0.693 × Vd ÷ CL |
| Loading dose | LD = Css × Vd ÷ F |
| Maintenance rate | Rate = CL × Css ÷ F |
Questions
0 of 15 answered
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1. A drug has a half-life of 6 hours. What fraction of it is left after 18 hours?
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Answer: 12.5%. 18 hours is three half-lives: 50%, then 25%, then 12.5%.
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2. With repeated dosing, about how long does a drug take to reach steady state?
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Answer: 4 to 5 half-lives. It is about 94 to 97% of the way there after 4 to 5 half-lives, whatever the dose.
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3. A 500 mg IV dose gives an initial concentration of 10 mg/L. What is the Vd?
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Answer: 50 L. Vd = 500 mg ÷ 10 mg/L = 50 L.
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4. Vd is 40 L and half-life is 7 hours. What is the clearance, approximately?
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Answer: 4 L/h. k = 0.693 ÷ 7 = 0.099 per hour, and CL = 0.099 × 40 = about 4 L/h.
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5. Target concentration is 15 mg/L, Vd is 30 L and F is 1. What is the loading dose?
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Answer: 450 mg. LD = 15 mg/L × 30 L ÷ 1 = 450 mg.
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6. Clearance is 5 L/h and the target is 10 mg/L. What is the maintenance rate, with F = 1?
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Answer: 50 mg/h. Rate = 5 L/h × 10 mg/L = 50 mg/h.
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7. At equal doses, oral AUC is 40 and IV AUC is 80. What is the oral bioavailability?
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Answer: 0.5. F = 40 ÷ 80 = 0.5.
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8. A drug with extensive first-pass metabolism will usually have what?
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Answer: Low oral bioavailability. Much of the dose is metabolized before it reaches the systemic circulation.
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9. In first-order elimination, what is removed per unit of time?
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Answer: A constant fraction. A constant fraction is removed, so the half-life stays the same.
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10. Which drug is known for moving to zero-order kinetics at higher concentrations?
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Answer: Phenytoin. Phenytoin metabolism saturates, so a small dose increase can raise levels sharply.
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11. With first-order kinetics, what happens to steady-state concentration if the dose rate doubles?
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Answer: It doubles. Css is proportional to dose rate when clearance is constant. The time to reach steady state does not change.
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12. If clearance halves and Vd stays the same, what happens to half-life?
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Answer: It doubles. Half-life is proportional to Vd ÷ CL, so halving clearance doubles it.
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13. Grapefruit juice raises levels of some oral drugs mainly by inhibiting what?
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Answer: CYP3A4 in the gut wall. Inhibiting intestinal CYP3A4 reduces first-pass metabolism.
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14. Which is a well-known potent enzyme inducer?
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Answer: Rifampin. Rifampin induces several CYP enzymes and can lower levels of other drugs. The other three are inhibitors.
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15. Alkalinizing the urine increases excretion of a weak acid such as aspirin. Why?
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Answer: More of it is ionized and trapped in the urine. The ionized form is less able to cross back through the tubule, so more leaves in the urine.
For exam study only, not clinical guidance. Doses, reference ranges and protocols vary, so follow your course, your local policy and current references. Spotted a mistake? Tell us.
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